Metal oxide-coated SiC foam carriers for catalysts, and catalytic systems therefor

ABSTRACT

A catalyst carrier including a surface layer of oxygen-donating oxide, particularly cerine, is disclosed. A catalytic phase deposited on said carrier generally includes platinum and rhodium, thus forming a catalytic system for exhaust pipes, said catalytic phase being readily recoverable by simple leaching.

FIELD OF THE INVENTION

The invention concerns catalysts, in particular high surface areacatalyst supports, and more particularly supports of silicon carbide,SiC.

DESCRIPTION OF RELATED ART

The manufacture of SiC foams for the production of catalyst supports forthe treatment of exhaust gases is already known, in particular fromFrench patent application FR-A1-2 705 340.

Catalytic converters for internal combustion engines are usually formedfrom a ceramic support, typically of dense and compact cordierite. Thesupport is then treated to form a deposit of alumina which adheres tothe cordierite support and is known as a wash coat. An active catalyticphase is deposited on the alumina layer, which phase may be constitutedby a cerium oxide, in particular ceria (CeO₂), followed by platinum andrhodium.

The automobile industry is researching more effective and cheapercatalytic systems for exhaust silencers to satisfy the ever more strictrules and regulations relating to depollution of exhaust gases fromcombustion engines and, of course, to satisfy economic demands.

The problem is thus to develop a new generation of catalytic systemstarting from an SiC foam as described in French application FR-A1-2 705340, which catalytic system has both an improved technical performanceand an economic advantage.

SUMMARY OF THE INVENTION

A first aim of the invention is to provide a catalyst support based onan SiC foam.

A second aim of the invention is to provide a process for the productionof said support.

A third aim is to provide the corresponding catalytic system.

A fourth aim of the invention is to provide a process for the productionof said catalytic system.

In a first aspect of the invention, the catalyst support comprises anSiC material in its divided and porous state and is characterized inthat said material comprises a superficial layer of an oxygen donoroxide.

The SiC based material in its divided state is preferably a formedelement of SiC foam with a specific surface area of at least 5 m²/g, asdescribed for example in French patent application FR-A1-2 705 340 citedabove. The divided state of the SiC constituting the foam contributes tothe high specific surface area of the foam; further, the divided stateis essential to capture of the superficial oxygen donor oxide layer.

The term oxygen donor oxide means a metal oxide with a difference (t)from the stoichiometry of the global formula of said oxides. In otherwords, an oxide with global formula M_(x)O_(y) may produce oxides withformula M_(x)O_(y-t). Its role is to regulate the oxygen potentialduring catalytic reactions.

The oxygen donor oxide generally comprises metal oxides with at leasttwo oxidation numbers but which remain in the oxide form and do not formcarbides during the catalysed reactions which take place in thecatalytic converter of the present invention.

These oxides are generally those of metals from group 3 of the periodicclassification of the elements, i.e., Sc, Y, the lanthanides andactinides, preferably rare earth oxides and more particularly cerium,such as ceria CeO₂, or mixed oxides of rare earths such as misch metal.

The oxides can also be those of metals from both columns 7, 8, 9, 10 and11 and rows 5 and 6, for example oxides of Pt, Rh, Pd, which arehenceforth termed so-called precious metals.

We have, surprisingly, discovered that depositing a superficial layer ofan oxygen donor oxide, in particular cerium, can simultaneously achievethe following effects:

-   -   good direct adherence of the layer to the SiC support material        in its divided state;    -   a large increase in the specific surface area;    -   good adherence for the subsequent deposit of the catalytically        active phase.

The good adherence can be deduced from porosity measurements usingmercury (see FIG. 5 for ceria) which show that the oxide (ceria) isdeposited into all the porosity of the high specific surface area SiCsupport with a thickness which is of the order of several hundredths ofμm. As shown in FIG. 5, the SiC foam support has two ranges of porosity:macropores of 10 to 100 μm, and mesopores of 0.007 to 0.1 μm. Theaddition of ceria does not modify the macropores. In contrast,deposition of the indicated thickness of ceria reduces the dimensions ofthe mesopores from initial dimensions in the range 0.04 to 0.1 μm to0.007–0.02 μm.

Thus the divided SiC support, free of a wash coat, provides the specificsurface area and produces good adherence of the catalytic phase, whichcan result in a high, stable catalytic activity during use as the oxidelayer does not suffer any changes.

For comparison, it has been observed that impregnation of knowncommercially available cordierite supports without prior deposit of awash coat based on alumina leads to the formation of 200 to 700 μmclusters of cerium oxide which then cannot effectively disperse theactive catalytic phase; this is the reason in this case why priorformation of an alumina-based wash coat is indispensable to provide thespecific surface area and adherence of the catalytic phase.

It has also been observed that impregnation of commercially availableSiC foams with a low specific surface area (<2 m²/g) cannot fix morethan 3% by weight of ceria on the support, which prevents good catalyticactivity.

In particular, it has been observed that, as shown in FIG. 5, with anSiC support in the absence of a superficial oxygen donor oxide layer,the specific surface area was much lower than when such a layer waspresent. Further, it has been observed that the presence of saidsuperficial oxygen donor oxide layer alone serves to disperse an activecatalytic phase in a well divided state effectively without formation oflarge clusters of low catalytic activity.

In accordance with the invention, the oxygen donor oxide/SiC ratio (inparticular for cerium) is generally in the range 10% to 50% by weightwith respect to the SiC, preferably 20% to 40%, for the more usualmetals cited above. A reduction in catalytic activity has been observedfor an oxide content of less than 10% by weight and the pores of the SiCfoam become blocked for contents of more than 50% by weight.

This ratio can go down to 1% for some oxides, in particular those of thegroup of metals known as precious metals.

In accordance with the invention, the superficial oxygen donor oxidelayer can comprise a stabilising agent for said oxide to avoid the riskof sintering, the content of which is in the range 0.1% to 5% by weightwith respect to the oxide.

The stabilising agent can be selected from the elements Si, Al, Mg, Ca,Zr, La and/or oxides thereof: silica, alumina, magnesia, lime, zirconia,and lanthanum oxide.

As mentioned above, the presence of a superficial oxygen donor oxidelayer in accordance with the invention has been observed to lead to alarge increase in the specific surface area of the support of theinvention. Thus the specific surface area of the final support of theinvention is generally in the range 1.5 to 4 times that of the startingSiC material in its divided state.

This specific surface area is related to the pore spectrum of thesupport, as shown in FIG. 5 (curve II) for ceria: the bimodal porespectrum has a macroporosity range of 5 to 100 μm and a mesoporosityrange of 0.007 to 0.5 μm. Other spectra, which may be shifted to smallerdimensions, can be obtained starting from a finer SiC support.

In a second aspect, the invention provides a process for the productionof the support of the invention, in which:

-   -   a) a solution of a precursor of the oxygen donor oxide is        prepared;    -   b) said SiC material in its divided state is impregnated with        said solution;    -   c) the excess of said solution is separated;    -   d) said impregnated material is calcined at a temperature which        is at least equal to the decomposition temperature of said        precursor but lower than the sintering temperature of said        oxide, to form a layer of said oxide on said SiC material in its        divided state.

The solution is preferably an aqueous solution, preferably a nitrate.For cerium, the precursor is a cerous salt which is soluble in water,with an oxidation number of +III, preferably a cerous nitrate: calciningthus produces ceria.

In step c) of the process, the excess of said solution is advantageouslyseparated by centrifugation.

In general, steps a), b) and c) of said process are carried out at acontrolled temperature, the temperature being in the range 15° C. to 70°C.

The oxygen donor oxide/SiC ratio can be varied by adjusting theconcentration of the oxide precursor solution in step a), preferablybetween 20% by weight and the solubility limit of the precursor in saidsolution, and/or, in step c), by separating the excess to a greater orlesser extent.

In a third aspect, the invention provides a catalytic system comprisingan active catalytic phase deposited on the catalyst support of theinvention.

The active catalytic phase normally comprises a metal or a mixture ofprecious metals such as platinum and/or palladium, and rhodium, to forma catalytic system for exhaust silencers, but it can contain othercatalytically active elements. In the case of platinum/rhodium, theplatinum content in the catalytic system is in the range 0.05% to 3% byweight with respect to the weight of the catalytic system, and therhodium content is in the range 0.01% to 3% by weight.

In a fourth aspect, the invention provides a process for the productionof said catalytic system of the invention, comprising the followingsteps:

-   -   1) depositing said active phase comprising, for example,        platinum and rhodium, on said catalytic support using any known        method from a solution of precursors;    -   2) separating said catalytic support from the excess of said        solution, preferably by drying;    -   3) heat treating said catalytic support on which said precursor        solution has been deposited, at a temperature which is greater        than the decomposition temperature of said precursors of        platinum and rhodium, to form said catalytic phase.

The platinum precursor solutions are preferably solutions ofchloroplatinic acid H₂PtCl₆ or platinum II amine Pt(NH₃)₂Cl₂ and inwhich the rhodium precursor solutions are solutions of a rhodium saltwith oxidation number +III selected from chlorides, nitrates andsulphates.

The platinum concentration in said precursor solution is in the range 20mg/l to 60 g/l and the rhodium concentration in said precursor solutionis in the range 20 mg/l to 60 g/l. The ratio (by weight) of Pt/Rh to theactive phase deposited on the support is generally in the range 3 to 6.

Unexpectedly, it has been discovered that the catalytic metal phase isbetter dispersed when deposition on the catalytic support is carried outby adsorption of the platinum and rhodium in the form of anionic orcationic species in solution, for which purpose the surface of saidsupport will have been respectively positively or negatively charged onsaid catalytic support by selecting a pH for said solution which islower or higher than the isoelectric point of said hydrated oxygen donoroxide. The isoelectric point is defined as the pH corresponding to theabsence of charge at the surface of the hydrated oxygen donor oxide insolution.

As shown in FIGS. 6 b and 6 c for cerium, which show the surface chargeas a function of pH, it is clear that, in contrast to the case of asimple SiC support, an SiC support which has been treated in accordancewith the invention by deposition of ceria leads (FIG. 6 c) to a supportfor which the isoelectric point is close to 6, and it is possible toselect one or another adsorbed anionic or cationic species containingthe precious metal, while a simple SiC support, with an isoelectricpoint of the order of 2, presents a much narrower breadth of choice.

In order to vary the weight content of platinum and/or rhodium, theconcentrations of the platinum and/or rhenium precursors in theprecursor solutions are varied and/or, in step 2) of the process, theexcess is separated to a greater or lesser extent.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1 a to 1 d are photographs of the SiC material used in theexamples. The scale shown on the right hand side is 500 μm for FIG. 1 a,20 μm for FIG. 1 b, 10 μm for FIGS. 1 c and 5 μm for FIG. 1 d.

It should be noted that FIG. 1 c relates to a “planar”portion while FIG.1 d relates to a combination of “planar” or “cylindrical” surfaces ascan be seen in the enlargement in FIG. 1 b.

FIG. 2 is an X ray fluorescence spectrum showing the peak correspondingto the element Si.

The remaining figures relate to the catalyst support of the inventionfor ceria:

FIGS. 3 a and 3 b are analogous to FIGS. 1 c and 1 d.

FIG. 4 is an X ray fluorescence spectrum showing the presence of a peakcorresponding to the element Si and a lower peak corresponding to theelement Ce.

FIG. 5 is a pore spectrum with the pore diameter in μm along theabscissa and the pore volume in cm³/g up the ordinate:

-   -   curve 1, with open squares, with higher peaks, is for an SiC        support before deposition of the oxygen donor oxide layer;    -   curve II, with solid squares, is for the same support coated        with ceria as described in the examples; its distribution        includes very small pores, in particular around 0.01 μm, which        generates a very high specific surface area.

FIGS. 6 b and 6 c are curves showing the surface charge (up theordinate—arbitrary units) as a function of pH (along the abscissa).

FIG. 6 b relates to an SiC support and FIG. 6 c relates to an SiCsupport which has been treated in accordance with the invention, withdeposition of ceria.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

If the present invention is compared with the closest and bestperforming currently available technique constituted by cordieritesupports coated with an alumina layer on which a ceria layer and anactive catalytic phase are deposited, and the corresponding catalyticsystems, in particular those using Pt—Rh, the following principaladvantages of the invention can be observed:

-   -   because the support of the invention contains no alumina (wash        coat), which is known to undergo gradual sintering at the        operating temperatures of the catalytic converter, thus reducing        its efficiency, the support of the invention ensures that the        catalytic system has a much longer lifetime due to the absence        of sintering, in particular in the case of ceria.    -   the particularly favourable hydrodynamics of the support for        catalysis of depollution reactions means that the catalytic        system of the invention, comprising for example Pt—Rh and        intended for the treatment of exhaust gases, has a volume which        is 30% lower for the same catalytic activity than the prior art        catalytic system with a cordierite support, which is of        particular advantage as regards the quality/price ratio.    -   finally, it has been observed that, surprisingly and        fundamentally with regard to recycling of materials, precious        metals such as Pt—Rh can be recovered more easily, more        economically and particularly effectively.

One characteristic of the invention is that the deposition of an activemetal on the support can be recovered by means of simple lixiviation bybringing the catalytic system of the invention into contact with a hotconcentrated acid solution (HCl, HNO₃, . . . ). This operation meansthat almost all of the precious metals, i.e., at least 98% thereof, canbe recovered, typically in less than a day. The solution obtained,further, is readily recyclable to carry out a fresh deposition of Pt—Rh,because of the high purity of the precious metals it contains.

As a comparison, with a prior art cordierite based catalytic systemcomprising an alumina-based wash coat, the cordierite support must becompletely dissolved because of the formation of very stable species ofPt and Rh, in particular Rh, with the alumina, which formationencourages the gradual sintering of the alumina, and thus results inincreased consumption of the reactant. In addition, the presence of thewash coat on the cordierite causes large filtration problems whenrecycling the precious metals.

EXAMPLES Example 1

A complete catalytic system was produced from a block of SiC foam with avolume of 1.4 1. The foam was produced as described in Example 5 (test4) of FR-A1-2 705 340. The SiC foam had a specific surface area of 14.2m²/g and a pore distribution (pore spectrum) as shown in FIG. 5 (curve1).

Ceria Impregnation Step

The SiC block was immersed in two liters of a 55% by weight aqueoussolution of hexahydrated cerous nitrate (Ce(NO₃)₃)₃,6H₂O). The ensemblewas kept immersed and degassed under partial vacuum for 10 minutes. Theblock was then separated from the solution, drained and dried bycentrifugation. The weight of the wet block impregnated with cerous saltwas 263 g. The deposited salt was thermally decomposed by slowly heatingthe impregnated block to 450° C. and maintaining that temperature for 3hours. At the end of that step, a catalyst support was obtained whichwas composed of an SiC foam with a ceria layer representing 18.6% of thetotal weight of the final support. The specific surface area was 24.5m²/g, i.e., 1.72 times the specific surface area of the startingsupport. The pore spectrum corresponded to curve II of FIG. 5 and had amaximum in the 0.007 μm–0.01 μm zone.

Impregnation of Active Phase Metals

The catalytic support obtained above (SiC foam and ceria) was thenimmersed in two liters of an aqueous solution containing 16.6 g/l ofplatinum in the form of chloroplatinic acid and 4.2 g/l of rhodium inthe form of rhodium nitrate. This ensemble was degassed under partialvacuum and kept immersed for ten minutes. The support was then separatedfrom the solution, drained and dried by centrifugation. The weight ofthe wet impregnated support was 251 g. The platinum and rhodium saltswere decomposed by heat treatment with a gradual rise in temperature to400° C. and maintaining that temperature for 3 hours.

A complete catalytic system was thus obtained which was constituted byan SiC foam coated with a ceria layer (18.6% of the total weight) whichacted as a support for the dispersed precious metals Pt and Rh (Pt:0.86% of the total weight, and Rh: 0.21% of the total weight).

Precious Metal Recovery Step

The catalytic system comprising the ceria layer and the precious metalcoating was coarsely ground then brought into contact with a boilingmixture of 50% of 53% nitric acid and 50% of 36% hydrochloric acid. 36%hydrochloric acid was added in successive fractions until the nitrateshad completely decomposed. After 5 hours, a suspension of SiC in asolution of chloroplatinic acid and rhodium chloride was obtained. Aftercooling, easy filtration and washing the solid residue, recovered as awhole, X ray fluorescence measurements showed that the residue containedless than 0.1% of platinum; the solution (filtrate) recovered bylixiviation contained 97% of the initially deposited platinum (thispercentage could have been increased by improving the washing); theresults obtained for the rhodium were equivalent: the solution wasdirectly recyclable to form a new deposit.

It should be noted that in the case of a cordierite support, recovery ofthe precious metals by lixiviation using the same acids led to partialdissolution of the cordierite and great difficulties with filtration(presence of colloids) due to the existence of the wash coat which didnot allow a material balance to be obtained.

Example 2

This example illustrates the process of the adsorption of Pt and Rh inthe form of anionic species to allow better distribution of the metalcatalytic phase by means of controlling the pH during the SiC foamimmersion step.

A catalytic support composed of an SiC foam and a ceria layer (24.9% byweight) was prepared as described in Example 1, up to the ceriaimpregnation step. The specific surface area of the support was 27.9m²/g. A 2.5 g sample was removed from this support.

Precious Metal Adsorption Step

The sample was immersed for 24 hours in 100 ml of a solution which wasstirred and circulated in which the concentrations of Pt and Rh werefixed initially and in which the pH was regulated so as to obtainadsorption of anionic species on the positively charged support asfollows:

initial conditions:

-   -   Pt: 0.514 g/l (in the form of chloroplatinic acid)    -   Rh: 0.266 g/l (in the form of rhodium chloride)

pH conditions:

-   -   The pH was kept between 3.1 and 3.5 by addition of ammonia or        nitric acid.

The surface charge curves of FIGS. 6 b and 6 c show that, for a supportwhich was free of ceria (FIG. 6 b), it was not possible to obtain apositively charged surface for pH values of 3.1 to 3.5, the conditionswhich are necessary to obtain the desired anionic speciessimultaneously. In contrast, the complete SiC support with depositedceria could produce positive surface charges under these pH conditions(FIG. 6 c).

After 24 hours, the support was separated from the solution and simplydrained. Measurement of the Pt and Rh remaining in the solution (0.43g/l for Pt, 0.219 g/l for Rh) meant that the quantities deposited on thesupport could be calculated. The deposited salts were then thermallydecomposed by gradually increasing the temperature to 400° C. followedby maintaining that temperature for three hours.

A complete catalytic system had thus been produced, constituted by anSiC foam coated with a ceria layer (24.9% of the total weight) acting asa support for the precious metals Pt and Rh (Pt: 0.33%±0.02 of the totalweight, and Rh: 0.19%±0.01 of the total weight). The catalytic systemobtained had excellent catalytic activity compared to the commerciallyavailable systems using the same precious metals supported by cordieritewith a wash coat, and excellent resistance to sintering. This goodcatalytic activity, better than that obtained from Example 1, can beexplained by better dispersion obtained by bringing ionic interactionsinto play for depositing the precious metals. It was also observed thatthe precious metal distribution was very homogeneous despite the absenceof drying, which absence is generally the cause of accumulation ondecanting the catalytic phase. Further, in contrast to the impregnationstep of Example 1, this adsorption technique means that dilute recycledsolutions of Pt and Rh salts can be used.

After using the catalytic system, the support underwent lixiviation torecover the precious metals deposited, using the operating protocoldescribed in Example 1. Analysis of the lixiviate (after filtration andwashing) enabled the following lixiviation yields to be calculated:

$\begin{matrix}{\frac{{Pt}{\mspace{11mu}\;}{recycled}}{{Pt}\mspace{14mu}{initial}} = {100\underset{\_}{+}{2\%}}} \\{\frac{{Rh}\mspace{14mu}{recycled}}{{Rh}\mspace{14mu}{intial}} = {100\underset{\_}{+}{2\%}}}\end{matrix}$

Example 3

This example illustrates the production of a support of the inventioncomprising a mixed oxygen donor oxide layer comprising a mixture of CeO₂and ZrO₂, the latter acting as an oxygen donor and stabilising additive.

The support was produced from an SiC foam block with a volume of 1.4 1,the foam being manufactured as described in Example 5 (test 4) ofFR-A1-2 705 340. This foam had a specific surface area of 12.3 m²/g.

Ceria Impregnation Step

The SiC block was immersed in two liters of a 45% by weight aqueoussolution of hexahydrated cerous nitrate Ce(NO₃)₃,6H₂O and 8% by weightof pentahydrated zirconium nitrate Zr(NO₃)₄,5H₂O. The ensemble was keptimmersed and degassed under partial vacuum for 10 minutes. The block wasthen separated from the solution, drained and dried by centrifugation.The weight of the wet block impregnated with cerous salt was 252 g. Thedeposited salt was thermally decomposed by slowly heating theimpregnated block to 550° C. and maintaining that temperature for 3hours. At the end of that step, a catalyst support was obtained whichwas composed of an SiC foam with a mixed layer of ceria and zirconiacontaining 16.4% of ceria and 2.4% of zirconia, with respect to thetotal weight of the final support. The specific surface area was 23.5m²/g, i.e., 1.91 times the specific surface area of the startingsupport.

Such a support has the advantage of stabilising the surface during use.As described in Examples 1 and 2, this support can be covered withactive phase and treated for recovery thereof.

1. A catalyst support comprising a support of SiC foam in a divided andporous state having a specific surface area of at least 5 m²/g, with asurface of SiC having thereon a superficial layer comprising an oxygendonor oxide, wherein the superficial layer resides directly on thesurface of SiC without an intermediate wash coat.
 2. A support accordingto claim 1, wherein the oxygen donor oxide is an oxide of at least onemetal from group 3 of the periodic classification of the elements.
 3. Asupport according to claim 2, wherein the oxygen donor oxide is a ceriumoxide or an oxide of a mixture of rare earths.
 4. A support according toclaim 1, wherein the oxygen donor oxide is an oxide of at least onemetal found in both group 7, 8, 9, 10 or 11 and row 5 or 6, of theperiodic classification of elements.
 5. A support according to claim 1,wherein the oxygen donor oxide is present in an amount of more than 1%by weight with respect to the SiC support.
 6. A support according toclaim 1, wherein the superficial oxygen donor oxide layer comprises astabilizing agent for said oxide in an amount of 0.1% to 5% by weightwith respect to said oxygen donor oxide.
 7. A support according to claim6, wherein said stabilizing agent is selected from the group consistingof Si, Al, Mg, Ca, Zr, La, silica, alumina, magnesia, lime, zirconia,lanthanum oxide and mixtures thereof.
 8. A support according to claim 1,having a specific surface area in the range 1.5 to 4 times that of saidSiC foam in its divided state.
 9. A support according to claim 1, havinga bimodal pore spectrum with a macroporosity range of 5 to 100 μm and amesoporosity range of 0.007 to 0.5 μm.
 10. A process for the productionof a support according to of claim 1, comprising the steps of: a)preparing a solution of a precursor of the oxygen donor oxide; b)impregnating said SiC foam in its divided state with said solution; c)separating excess of said solution; and d) calcining said impregnatedfoam at a temperature which is at which said precursor decomposes butlower than a temperature at which said oxide sinters, to form a layer ofsaid oxide on said SiC foam in its divided state.
 11. A processaccording to claim 10, wherein said solution is an aqueous solution andsaid precursor is a cerium salt which is soluble in water.
 12. A processaccording to claim 11, wherein said salt is a cerous nitrate.
 13. Aprocess according to claim 10, wherein the excess of said solution isseparated by centrifugation.
 14. A process according to claim 11,wherein the cerium oxide/SiC ratio is varied by at least one of thesteps of: in step a) of the process, changing the concentration of thecerium oxide precursor in said solution between 20% by weight and thesolubility limit of said precursor in said solution, and in step c) ofthe process, changing the degree to which said excess is separated. 15.A catalytic system comprising an active catalytic phase comprising oneor more active catayltic elements, deposited on said catalyst supportaccording to claim
 1. 16. A catalytic system according to claim 15,wherein said active catalytic phase is selected from the groupconsisting of platinum, palladium, rhodium and mixtures thereof, to forma catalytic system for the treatment of exhaust gases.
 17. A catalyticsystem according to claim 16, wherein said catalytic system containsplatinum in the range 0.05% to 3% by weight, and rhodium in the range0.01% to 3% by weight.
 18. A catalytic system according to claim 16,containing Pt and Rh in a weight ratio in the range of 3 to
 6. 19. Aprocess for the production of said catalytic system according to claims15, comprising the steps of: a) preparing a solution of a precursor ofthe oxygen donor oxide; b) impregnating said SiC foam in its dividedstate with said solution; c) separating excess of said solution; d)calcining said impregnated foam at a temperature which is at which saidprecursor decomposes but lower than a temperature at which said oxidesinters, to form a layer of said oxide on said SiC foam in its dividedstate; e) depositing said active phase comprising at least one activecatalyst element from a precursor solution thereof; f) separating saidcatalytic support from excess precursor solution; g) heat treating saidcatalytic support on which said precursor solution has been deposited,at a temperature which is greater than that at which the precursordecomposes, to form said active catalytic phase.
 20. A process accordingto claim 19, wherein the active elements are platinum and rhodium.
 21. Aprocess according to claim 19, wherein, in step e), said deposition iscarried out by adsorption of the at least one active element in the formof anionic or cationic species in solution on said support, the surfaceof which has been respectively positively or negatively charged byselecting a pH for said solution which is lower or higher than theisoelectric point of the hydrate of said oxygen donor oxide.
 22. Aprocess according to claim 19, wherein the at least one active elementis varied in weight content by at least one step of: changingconcentrations of said precursor in the solutions and, changing theamount of separation of said excess.
 23. A process for recovering thecatalytic phase based on precious metals in the catalytic system ofclaim 15, comprising treating said catalytic system by coarse grinding,lixiviation using a hot concentrated acid, then filtering to obtain asolution of at least one catalyst metal.
 24. A support according toclaim 2, wherein the metal is a rare earth metal.
 25. A supportaccording to claim 4, wherein the at least one metal is selected fromthe group consisting of Pt, Rh and Pd.
 26. A support according to claim5, wherein the oxygen donor oxide is present in an amount of 10 to 50%by weight.